One of the most popular methods of assembling parts without mechanical fasteners is to join surfaces of the respective parts with an intermediate substance, sometimes called a bonding material. Although there are many such bonding materials, the types discussed in this application are those which are liquefied by heat for the bonding. A problem is to select a bonding material and apply the same to suit a method by which parts are assembled to acquire joints having desired characteristics.
A challenging task is to bond an often delicate, electronic chip with precise registration to a substrate and with a joint having good conductivity of electricity and operating heat. One typically starts by metallizing the surfaces to be joined. Then a metal or a metal alloy is typically selected which will react with the surface metals to form an alloy having the conductivity and/or other desired characteristics. A problem is to liquefy and uniformly distribute the bonding material over the surfaces to be joined. Another problem is to obtain a uniform and desired reaction time over the joint and between the bonding material and the surface metals.
A desirable method of achieving uniform distribution is to first form a solid workpiece of bonding material (called a "preform") and place the same between cold surfaces prior to any heating. Such method assures a uniform and desired reaction time providing heat is uniformly applied and uniformly and timely removed. Oxidation of surfaces and bonding material at elevated temperatures is mitigated because the cold joining tends to mitigate air in a joint during a later heating step. A problem is that uniform heating is often done in a chamber away from an assembly station. Many electronic chip assemblies are so tiny, precise and delicate that clamping and moving steps risk damage to chips and their respective registration to substrates.
Uniform heating at an assembly station is often achieved by heating a substrate (rather than a vulnerable chip). A problem is to control liquefication of a preform of bonding material while the preform and a chip are being registered to a site on a substrate. If heat is avoided until after registration of the preform and chip, respectively, an unacceptable waiting period is experienced during the heating step. If the heat is applied before registration, the preform liquefies and oxidizes while the chip is being registered to the preform.
Accordingly, it is desirable to provide new and improved expedients for bonding a first article such as a chip to a substrate utilizing a second article such as a preformed workpiece of bonding material. It is preferred to engage a chip and then engage the preform to the chip, before contacting either article to a heated substrate.